The present invention generally relates to a biphase shift keying (BPSK) modulation circuit for digital telecommunication systems, in particular to a constant envelope BPSK modulator for use in driving a power amplifier, inter alia, of C-class.
One of the known BPSK modulators together with its transmission circuitry is disclosed in FIG. 1, which the circuit includes a data generator 1 providing a given digital signal, a low-pass filter (LPF) 2 suppressing high-frequency components of the digital signal, and a modulator 3 for modulating in phase the filtered output from the LPF into a carrier-wave signal as shown in FIG. 2, which illustrates a waveform of the modulated output signal from the modulator 3. This modulated signal is delivered to a band-pass filter (BPF) 4 and then amplified in a C-class power amplifier 5. Antenna (ANT) receives the amplified signal for transmission into the air. This type of BPSK modulator, as shown in FIG. 1, adopts a modulation system in which a phase of the carrier-wave (CR) is instantaneously shifted in 0.degree. and 180.degree. in dependence upon the digital signal from the data generator, during modulation of the digital signal to the carrier-wave. Thus, when the carrier-wave (CR) passes through the BPF, a phenomena occurs in that its amplitude decreases at a given zero-crossing position, as shown by a reference symbol A of FIG. 2, during the shifting of the carrier-wave in 0.degree. and 180.degree.. This is because the frequency in the zero-crossing position gets out of a passing frequency band in the BPF. Therefore, in the power amplifier 5, operating in C-class level, adequate amplification of the modulated signal may not be provided at the zero-crossing positions, thereby resulting in distortion of output signals.